When tightening screw joints, especially the typical screw joints of the motor vehicle industry and similar volume production, a homogeneous and well defined clamp force is the target in a majority of the applications. Various methods have been developed to control the clamp force by the indirect means of torque, angle and time measurements, i.e. the measurable parameters of a typical screw joint tightening. During the design phases of a screw joint theoretical calculations complemented with full scale tests or simulations help to decide which tightening method should be used.
A modern screw joint tightening system, like the PowerMACS-system marketed by Atlas Copco, comprises one or more electric nutrunner spindles, and a monitoring and control system including a process computer or system intelligence. Each nutrunner spindle is equipped with one or two torque transducers and one or two angle encoders. The system measures the motor current and the motor speed in various ways.
Various methods have been developed for controlling, or eliminating to the largest possible extent, the negative influence of the friction between thread and contact surfaces, screw geometry and material properties. All these methods are based on software algorithms which form part of the controller programs and make it possible to control and compensate for most of the encountered variations in the production process.
A simulation program is used in the above mentioned screw joint tightening system, PowerMACS, for simulating production results assuming certain hardware, joint characteristics and variable input parameters and using the same software parameters as used in the actual screw joint tightening performed by the Power MACS. The results of the simulations are reported as production statistics normally evaluated by the QA-engineers of the industry. With the representative screw joint data, system hardware and software the simulation program has the advantage of being able to calculate also the screw joint clamp force which is the most wanted information. Under production conditions information of the achieved clamp force is obtainable indirectly only, which is a shortcoming.